Particle detection system

ABSTRACT

This invention provides a design to process a large range of detection beam current at low noise with a single detector. With such a design, the detection system can generate up to 10 10  gain and maximum signal output at more than mini Ampere (mA) level. A condenser lens is configured to increase bandwidth of the detector that scan speed can be enhanced.

FIELD OF THE INVENTION

The present invention relates to a particle detection system, and moreparticularly to an electron detection device used for an E-beam system.

BACKGROUND OF THE INVENTION

Particle detection systems are used or applied in many facilities, suchas electron beam system, scanning electron microscope (SEM), focused ionbeam (FIB), mass spectrograph, or other facility necessary to detectparticles with or without charge, photons.

Electron beam wafer inspection tool (EBWIT) needs large range of beamcurrent to meet both high throughput (HT) mode and high resolution (HR)mode, in which the HT mode is driven by productivity and the HR mode isdriven, on the opposite side, by design rule which now continuesshrinking. The EBWIT usually uses several tens to several hundreds nA(nano Ampere) beam current for HT mode and uses several to several tensof pA (pico Ampere) beam current for HR mode. This requires the electrondetector used in the EBWIT can handle the signal beam current (SE+BSE)from several tens of pA to several hundreds of nA. This requirement is ahuge challenge for any current electron detection system.

Nowadays, there is no EBWIT that can be operated under both HT mode andHR mode; basically, HT mode is more popular due to the majority of theEBWIT detectors is SPD (semiconductor photodiode detector). For the HRmode, received detected current from specimen is too small for the SPDthat output signal current, with about 2000 gain from the input signalcurrent, is still a long way off to handle for the EBWIT.

Currently, semiconductor photo diode (SPD) is widely used for the EBWITdue to its low noise and potential of handling large beam currentnature, such as several mA (mini Ampere). However, the gain obtainedfrom SPD is very low (˜2000 @ 10 KV) for pA detection current and noteasy to be increased.

High gain, such as 10⁴-10⁷, can be acquired from using Phosphor-PMT(phosphor multiplier tube) system and MCP (micro channel plate)detectors. However, most PMT and MCP can only output maximal several uAsignal current, which cannot meet the requirement of large beam currentat low noise level, because the dynode in PMT limits the output currentand the gain variation is too large when PMT dynode operates at lowvoltage. Further, although the Phosphor-PMT system can achieve highgain, but high bandwidth scintillator usually contributes high levelwhite noise. However, if the image-average technique is used toeliminate the white noise, the throughput will be seriously hurt.

According to the above discussion, no detector system can output signalin the range from pA to mA. Thus, it is an important topic to formulatea particle detection system that can have a large range of gain andoutput signal current with low noise to meet both HT and HR modesrequirement.

Further, in order to handle such a situation, Wang et al. suggested aBi-Type detection system in U.S. patent application Ser. No. 12/715,766filed on Mar. 2, 2010.

SUMMARY OF THE INVENTION

The present invention is directed to a particle detection system, whichcan detect charged or uncharged particles.

The present invention is also directed to an electron detection system,which gain of both electron detection system and particle detectionsystem can be increased up to 10¹⁰ with mA output signal current.

The present invention is also directed to a scanning electron microscopecan be applied as EBWIT to meet both HT mode and HR mode requirements.

In one embodiment, the particle detection system includes a chargedparticle multiplier device for receiving original particles to bedetected and generating a plurality of multiplied charged particles toobtain a first level gain, a charged particle detection device forreceiving the plurality of multiplied charged particles and generatingoutput signal corresponding to the original particles, and a condenserlens is configured between the charged particle multiplier device andthe charged particle detection device to converge the plurality ofmultiplied charged particles into the charged particle detection device,wherein an electric potential is applied between the charged particlemultiplier device and the charged particle detection device toaccelerate the plurality of multiplied charged particles to arrive thecharged particle detection device to obtain a second level gain.

The condenser lens can be an electrostatic lens. Gain of the particledetection system can be adjusted by an electric field from theelectrostatic lens, wherein the electric field can be ranged from 5 KVto 15 KV. The particle detection system further comprises a metal meshconfigured to said charged particle multiplier device, and an energyfilter between the charge particle multiplier device and the metal mesh.The particle detection system further comprises a means for driving theoriginal particles from the metal mesh to the energy filter, wherein themeans for driving the original particles is draft tube structure.

The charged particle detection device can be a semiconductor photodiode. If the original particles are original charged particles, thecharged particle multiplier device can be a microchannel plate. If theoriginal charged particles are electrons, the charged particlemultiplier device can be an electron multiplier.

In another embodiment, this invention provides an E-beam waferinspection tool, which comprises an electron gun for emitting a primarybeam, a primary beam condenser lens for condensing the primary beam, anobjective lens for receiving the primary beam from the primary beamcondenser lens and focusing the primary beam on to a surface of aspecimen, a particle detection system for receiving particles emanatingfrom the surface of the specimen, and means for generating images fromthe output signal. The particle detection system includes a chargedparticle multiplier device for receiving original particles to bedetected and generating a plurality of multiplied charged particles toobtain a first level gain, a charged particle detection device forreceiving the plurality of multiplied charged particles and generatingoutput signal corresponding to the original particles, and a condenserlens is configured between the charged particle multiplier device andthe charged particle detection device to converge the plurality ofmultiplied charged particles into the charged particle detection device,wherein an electric potential is applied between the charged particlemultiplier device and the charged particle detection device toaccelerate the plurality of multiplied charged particles to arrive thecharged particle detection device to obtain a second level gain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram for illustrating particle detection systemaccording to an embodiment of the present invention.

FIG. 2 is a schematic diagram for illustrating MCP inner structure.

FIG. 3 is a schematic diagram for illustrating how one electron isamplified in one of the MCP.

FIG. 4 is a schematic diagram for illustrating electron detection systemaccording to an embodiment of the present invention.

FIG. 5 is a schematic diagram for illustrating particle detection systemaccording to another embodiment of the present invention.

FIG. 6 is a schematic diagram for illustrating an electrostatic lens tocondense multiplied charged particles in the particle detection systemaccording to an embodiment of the present invention.

FIG. 7 is a schematic diagram for illustrating a magnetic lens tocondense multiplied charged particles in the particle detection systemaccording to an embodiment of the present invention.

FIG. 8 is a schematic diagram for illustrating an energy filter anddriving means applied to the particle detection system according to anembodiment of the present invention.

FIG. 9 is a schematic diagram for illustrating an electron detectionsystem according to an embodiment of the present invention.

FIG. 10 is a schematic diagram for illustrating an electron detectionsystem applied to an E-beam wafer inspection tool.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The present invention provides a particle detection system. For completeunderstanding of the present invention, the following description willdescribe in detail the components. The present invention is not limitedby the specified particulars of the particle detection system that arefamiliar to persons skilled in the art. In addition, well-knowncomponents are not described in detail so as to avoid any additionallimitation. The preferable embodiments of the present invention aredescribed in detail. In addition to the detailed descriptions, thepresent invention also can be applied to other embodiments. Therefore,the scope of the present invention is not limited, and is dependent onthe following claims.

This invention applied an electric field between a charged particlemultiplier device for receiving original particles to be detected and acharged particle detection device for receiving the plurality ofmultiplied charged particles in a particle detection system, in whichthe charged particle multiplier device generates a plurality ofmultiplied charged particles to obtain a first level gain, and thecharged particle detection device generates an output signalcorresponding to the original particles. A condenser lens is configuredbetween the charged particle multiplier device and the charged particledetection device to condense or converge the plurality of multipliedcharged particles into the charged particle detection device. Thecharged particle detection device, by adjusting the field appliedbetween the charged particle multiplier device and the charged particledetection device, provide second gain for the charged particlesmultiplied by the charged particle multiplier device.

The signal particles to be detected are converted to charged particlesin the charge particle multiplier device and multiplied, and the gaincan be adjusted by adjusting the power applied on the charged particlemultiplier device. The charge particle multiplier device can be amicrochannel plate (MCP) if the signal particles, or original particles,are charged particles, in some cases electrons. The charged particlemultiplier device can be, but is not limited to, microchannel plate(without anode) for charged particle detection, electron multiplier(without anode) for electron detection, phosphor+ photo cathode+electron multiplier or MCP plate (without anode) for photon detection,etc.

The charge detection device can be, but is not limited to, semiconductorphotodiode (SPD), such as PiN, NiP etc. in most cases. Bandwidth of theSPD can be enhanced by using the condense lens, because multipliedcharged particles are weakly focused, or condensed by the condense lensand a smaller area of the SPD can be used to received the multipliedcharged particles. The function of the charged particle detection deviceis to acquire second gain for the charged particles, and to generate alarge output signal current (>0.1 mA) with low noise level. The secondgain can be adjusted by changing the high voltage acceleration fieldbetween the charge particle multiplier device and the charge particledetection device.

A metal mesh may be placed in the front of the charged particlemultiplier device, for example electron multiplier device, to preventthe leaking of the field applied on the electron multiplier device andattract the multiplied charged particle to the detector if used fordetecting charged particles.

An energy filter can be configured in front of the charged particlemultiplier device to filter out some signal electrons without enoughenergy. A driving means, for example can be sidewall potential, or drafttube structure, will force signal electrons from mesh to energy filteror MCP without bombarding to the sidewall.

Please refer to FIG. 1, wherein a particle detection system 100comprises a charged particle multiplier device 110 for receivingoriginal particles to be detected and then generating a plurality ofcharged particles and a charged particle detection device 130 forreceiving the plurality of charged particles and generating an outputsignal corresponding to the original particles through a signal outputinterface 132. An electric field of electric potential within a drifttube type housing 120 can be applied between the charged particlemultiplier device 110 and the charged particle detection device 130 toaccelerate the plurality of charged particles to arrive the chargedparticle detection device 130. The original particles in this inventioncan include photons, and charged particles, wherein the chargedparticles may be ions, electrons, or other particle with positive chargeor negative charge.

Original particles to be detected are converted into charged particles,such as electrons, by the charged particle multiplier device 110 if theoriginal particles are not charged. The charged particle multiplierdevice 110 for detecting the original particles, in one embodiment, caninclude a microchannel plate (MCP) (per se without anode), wherein theMCP is a two-dimensional sensor that can detect electrons, ions, UVrays, X-rays or gamma rays, and amplifies the detected signal. Detailsof the MCP can be referred to, for example the publication by Goldsteinet al., “Scanning Electron Microscopy and X-Ray Microanalysis,” SecondEdition, published by Plenum, 1992, Chapter 4, and can be referred toFIG. 2 and FIG. 3, wherein a schematic structure of a microchannel plate200 is illustrated in FIG. 2 and a schematic structure of how oneelectron is amplified in one of the a plurality of microchannels isillustrated in FIG. 3. A plurality of microchannels is arrayed within aplate as shown in FIG. 2, wherein each channel 210, for example, willpass through the plate.

In FIG. 3, one electron 10 is injected into one channel 210 as inputelectrons 212, and then is reflected and amplified through the channel210. A potential gradient is established along the channel 210 when avoltage is applied between input and output sides of the MCP. Multiplesecondary electrons are emitted when one electron 10 enters a channel210 from the input side and strikes its inner wall. These secondaryelectrons are accelerated by the potential gradient to draw parabolictrajectories that are determined by their inner velocities. They thenstrike the opposite wall in the channel 210 causing further secondaryelectrons to be emitted. The electrons in this way travel towards theoutput end while striking the inner wall of the channel 210 repeatedly.As a result, a large number of exponentially increased electrons asoutput electrons 214 are extracted to the charged particle detectiondevice 130 by the electric field or potential between the chargedparticle multiplier device 110 and the charged particle detection device130 from the output side.

The charged particle multiplier device 110 for detecting electrons, inone embodiment, can include an electron multiplier (per se or withoutanode), wherein the electron multiplier uses a vacuum-tube structurethat multiplies incident charges. In a process called secondaryemission, a single electron can, when bombarded on secondary emissivematerial, induce emission of roughly 1 to 3 electrons. If an electricpotential is applied between this metal plate and another one, theemitted electrons will accelerate to the next metal plate and inducesecondary emission of still more electrons. This can be repeated anumber of times, resulting in a large shower of electrons allaccelerated to the charged particle detection device 130 by the electricfield or potential between the charged particle multiplier device 110and the charged particle detection device 130.

The charged particle multiplier device 110 for detecting photons, in oneembodiment, can include phosphor, a photocathode, and a microchannelplate or an electron multiplier, wherein the photocathode is used fortransferring photons into charged particles, most electrons. Whenphotons strike onto a surface of the photocathode, a negatively chargedelectrode in a light detection device, a plurality of electrons areemitted and then accelerated to the charged particle detection device130 by the electric field or potential between the charged particlemultiplier device 110 and the charged particle detection device 130. Theelectron emission, can be current amplifier, from the photocathode isthen forward to the electron multiplier for multiplying incidentcharges.

One important function of the charged particle multiplier device 110 isto acquire the first level gain, and the charged particle detectiondevice 130 is to acquire a second level gain. Further, the function ofthe charged particle detection device 130 can generate a large outputsignal current (more than 0.1 mA) with low noise level. The chargedparticle detection device 130, in one embodiment, can be semiconductorphotodiode detector (SPD), such as PiN photodiodes.

The drift tube type housing 120 between the charged particle multiplierdevice 110 and the charged particle detection device 130 is used forconfining charged particles generated by the charged particle multiplierdevice 110. The electric field or potential applied between the chargedparticle multiplier device 110 and the charged particle detection device130 can be high voltage, such as ±1 KV to ±20 KV or above/below, toaccelerate the charged particles. Because gain of the semiconductorphotodiode detectors depends on landing energy of the charged particles,adjusting this applied electric field or potential can increase gain ofthe charged particle detection device 130 significantly, which isimpossible in the prior arts. Further, white noise issue in thePhosphor-PMT does not appear in the design.

The output signal in the signal output interface 132, forward to anamplifier in which transferred from current signal to voltage signal.The particle detection system 100 obtains maximum gain up to 10¹⁰ withmaximum output signal current more than mA level, in one embodiment with10 KV applied electric field or potential. This distinct performancewould be impossible for any prior particle detection system.

A metal mesh 112, which is an optional part, can be configured withinthe drift tube housing 120 and in front of an incident surface of thecharged particle multiplier device 110 for preventing the leakage of theelectric field or potential applied on the charged particles multiplierdevice 130.

One important application of the particle detection system is to detectelectrons. As shown in FIG. 4, an electron detection system 400 includesa metal mesh 112, a MCP 200, a drift tube type housing 120, a chargedparticle detection device 130, such as PiN photodiode detector, and asignal output interface 132. The MCP 200, in one embodiment, attractsoriginal particle, for example about 10 nA electrons and amplifies theseelectrons to a first level gain, for example 1000 times gain. And, theseamplified electrons will be accelerated by a 10 KV electric field orpotential within the drift tube type housing 120. The charged particledetection device 130, which in one embodiment uses SPD or PiN photodiodedetector, will obtain about 2000 times gain with about 20 mA outputsignal. Compared with a multi-stage MCP assembly with maximum 10 to 20uA output signal current (about 2K gain), this invention can providemaximum 10 to 20 mA output signal current (about 2 million gain). Thelater which is thousand times larger. In theory MCP has the capabilityto obtain 2 million gain, but output current of anode is limited to 20uA. Therefore, real value of multi-stage MCP assembly can only obtain 1Kto 2K gain for the 10 nA signal current.

Referring to FIG. 5, a particle detection device 500 is provided,wherein a lens 300 is configured to condense or converge the chargedparticles from the charged particle multiplier device 110 into thecharged particle detection device 130 in a preferred embodiment. Thelens 300 can be configured within or outside the housing 120, althoughin FIG. 5 the lens 300 is outside the housing 120. The metal mesh 112 isconfigured to receive the original particles, and as an incident surfacein front of the charged particle multiplier device 110. The signaloutput interface 132 contacts to the charged particle detection device130 and outputs signals to an image processing unit (not shown in FIG.5).

The original particles to be detected come to the metal mesh 112 andreach the charged particle multiplier device 110. Then, the originalparticles are converted to charged particles if the original particlesare electric neutral, and multiplied by the charged particle detectiondevice 130. The charged particles are next accelerated by an electricfield between the charged particle detection device 130 and the chargedparticle multiplier device 110, and also condensed or converged by thelens 300 to the charged particle multiplier device 110. The chargedparticles are detected and transferred as signals by the chargedparticle detection device 130, and the signals are output though thesignal output interface 132.

The charged particle multiplier device 110 can be MCP, and the chargedparticle detection device 130 can be SPD in a preferred embodiment.

The lens 300 in FIG. 5 can be electrostatic lens or magnetic lens.Please refer to FIG. 6, an electrostatic lens 310 is configured insidethe housing 120 to condense or converge the charged particles from thecharged particle multiplier device 110 to the charged particle detectiondevice 130. A simulation is shown how the charged particles arecondensed or converged by the electrostatic lens 310. Hence, only aportion of the charged particle detection device 130 will receive thecharged particles, so the SPD can use smaller area to receive the signalcharged particles to enhance bandwidth.

A magnetic lens 320 in FIG. 7 is configured outside the housing 120 tocondense or converge the charged particles. The magnetic lens 320includes an excitation coil 322 which provides magnetic field, and ayork 324 to guide the generated magnetic field to the opening of theyork 324 in FIG. 7. This provides another embodiment to condense orconverge the charged particle in the housing 120. The choice of usingelectrostatic lens or magnetic lens depends on the designer'srequirement.

If charged particles, such as electrons, are needed to be detected bythe particle detection system 100, an energy filter can be provided tofilter out some electrons with certain energy. As shown in FIG. 8 a andFIG. 8 b, an energy filter 810 is provided between the charged particlemultiplier device 110 and the metal mesh 112. In FIG. 8 a, if electronswith energy lower than a predetermined value, all electrons are blockedby the energy filter 810. In FIG. 8 b, if potential of the energy filter810 sets a value of −11002V, and the electrons have energy of 11000 eV,some electrons can pass through the energy filter 810 to the chargedparticle multiplier device 110.

A means 820 for driving charged particles, such as electrons, from themetal mesh 112 to the energy filter 810 is also provided in both FIG. 8a and FIG. 8 b. Because in common EBWIT or SEM, the metal mesh 112 andhousing 120 are set zero potential, the electrons reach the metal mesh112 will not to an energy barrier of the energy filter 810 with negativepotential. Therefore, the means 820 for driving the electrons is used toforce the electrons to the energy filter 810, not the housing 120. Ifthe original charged particles are positive, the driving means 820 isconfigured between the energy filter 810 and the charged particlemultiplier device 110 if the energy filter 810 is negative potential andthe charged particle multiplier device 110 is zero potential. Thedriving means 820, which will provide a little potential barrier to thecharged particles to the housing 120, such that the most chargedparticles will be forced to the energy filter 810. The driving means820, in one embodiment, can be drift tube structure.

In FIG. 8 a and FIG. 8 b, two simulations are shown that electrons, ascharged particles to be detected, after through the metal mesh 112, areforced by the driving means 820 and filtered out by the energy filter810 to the charged particle multiplier device 110. It is also noted thatthere is no lens to condense the electrons multiplied by the chargedparticle multiplier device 110 in FIG. 8 a and FIG. 8 b.

Referring to FIG. 9, an electron detection system 900 is provided, inwhich a housing 120 includes a metal mesh 112, an energy filter 810, acharged particle multiplier device 110, a charged particle detectiondevice 130 in sequence. A driving means 820 is configured at sidewall ofthe housing 120 between the metal mesh 112 and the energy filter 810 toforce electrons (as shown in arrow head lines in FIG. 9) from the metalmesh 112 to the energy filter 810. The electrons through the energyfilter 810 will reach and multiplied by the charged particle multiplierdevice 110. And then the multiplied electrons are accelerated by anelectric field between the charged particle multiplier device 110 andthe charged particle detection device 130, and condensed or converged bythe electrostatic lens 310 to the charged particle detection device 130.The charged particle detection device 130 then received the electronsand transfers to signals corresponding to the original electrons beforeentering the charged particle multiplier device 110. The signals arethen outputted through the signal output interface to an imageprocessing unit.

In FIG. 10, a EBWIT 1000 is provided, which comprises an E-gun 1010, acolumn 1100, and a stage 1130. The E-gun 1010 includes a field emissioncathode 1012 for providing E-beam 1002, an anode 1014 for applyingelectric field to the cathode 1012, a terminal electrode 1016 forproviding zero potential to the E-beam 1002, and a gun aperture 1017 forcutting off the E-beam 1002 with larger polar angles. The column 1100includes a condenser 1102 for weakly focusing the E-beam, and anelectromagnetic lens 1120 for focusing the E-beam on a specimen 1140.The electromagnetic lens 1120 includes an excitation coil 1124 forproviding magnetic field, and a york 1126, covering the excitation coil1124, will guide the magnetic field out of york 1126, which is alsocalled as pole-piece. At least a pair of electrodes 1128 inside the york1126 provides electric field to the E-beam as a retarding force. TheE-beam 1002 then bombarding the specimen 1140 and a plurality of signalelectrons, also known as secondary electrons, is emanated back anddeflected by a Wien filter 1122 to the detector 900.

The design of this invention can detect not only electrons or chargedparticles, but also photons or even other electric neutral particles.The charged particle multiplier device provides a first gain level and afunction to convert original particles into charged particles if theoriginal particles are electric neutral. An applied electric field orpotential between the charged particle multiplier device and the chargedparticle detection device can increase gain of the charged particledetection device, and a second gain level is obtained in this design.Further, by using the electric field or potential, the particledetection system can increase gain compared to or larger than themulti-stage MCP assembly, with thousand times larger output signalcurrent than the multi-stage MCP assembly. By using the condense lens inthe particle detection system, a smaller area of SPD can be used toreceive multiplied charged particles, and bandwidth of the SPD can befurther enhanced.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe invention as limited only by the appended claims.

1. A particle detection system, comprising: a charged particlemultiplier device for receiving original particles to be detected andgenerating a plurality of multiplied charged particles; a chargedparticle detection device for receiving the plurality of multipliedcharged particles and generating output signal corresponding to theoriginal particles; and a condenser lens is configured between saidcharged particle multiplier device and said charged particle detectiondevice to converge the plurality of multiplied charged particles intosaid charged particle detection device.
 2. The particle detection systemaccording to claim 1, wherein said charged particle detection device isa semiconductor photo diode.
 3. The particle detection system accordingto claim 2, wherein said original particles are original chargedparticles.
 4. The particle detection system according to claim 3,wherein said charged particle multiplier device is a microchannel plate.5. The particle detection system according to claim 3, wherein saidoriginal particles are electrons.
 6. The particle detection systemaccording to claim 5, wherein said charged particle multiplier device isan electron multiplier.
 7. The particle detection system according toclaim 6, wherein an electric field is applied between the chargedparticle multiplier device and the charged particle detection device toaccelerate the plurality of multiplied charged particles.
 8. Theparticle detection system according to claim 2, wherein said originalparticles are photons.
 9. The particle detection system according toclaim 8, wherein said charged particle multiplier device includesphosphor, photo cathode, or a microchannel plate.
 10. The particledetection system according to claim 6, wherein the condenser lens is anelectrostatic lens.
 11. The particle detection system according to claim10, wherein gain of the particle detection system can be adjusted by anelectric field from the electrostatic lens.
 12. The particle detectionsystem according to claim 11, wherein the electric field can be rangedfrom 5 KV to 15 KV.
 13. The particle detection system according to claim12, further comprising a metal mesh configured to said charged particlemultiplier device.
 14. The particle detection system according to claim13, further comprising an energy filter between the charge particlemultiplier device and the metal mesh.
 15. The particle detection systemaccording to claim 14, further comprising a means for driving theoriginal particles from the metal mesh to the energy filter.
 16. Theparticle detection system according to claim 15, wherein the means fordriving the original particles is draft tube structure.
 17. An E-beamwafer inspection tool, comprising: an electron gun for emitting aprimary beam; a primary beam condenser lens for condensing the primarybeam; an objective lens for receiving the primary beam from said primarybeam condenser lens and focusing the primary beam on to a surface of aspecimen.
 18. The E-beam wafer inspection tool according to claim 17,further comprising means for receiving particles emanating from thesurface of the specimen.
 19. The particle detection system according toclaim 1, further comprising means for generating images from the outputsignal.